It is conventionally known that the polarization state of light randomly fluctuates in optical fiber transmission due to asymmetry of a core shape formed in fiber production or the application of stress in a lateral direction. Therefore, when a receiving system that is sensitive to the polarization state is used, the random polarization fluctuation needs to be tracked at all times.
In currently commercialized digital coherent optical receivers, polarization tracking is realized through the use of “digital signal processing (DSP)”. This enables long-distance communication over a distance as long as approximately 8,000 km.
Meanwhile, a “self-homodyne optical transmission system” that transmits an optical carrier signal together with an optical modulation signal through polarization multiplexing is known as a simple method of digital signal processing. In this self-homodyne system as well, some polarization tracking methods using DSP have been proposed, and particularly, a “blind polarization tracking algorithm” that tracks the polarization state without using a symbol determination result is practically important in terms of downsizing the circuit scale.
FIG. 5 is a diagram illustrating a schematic configuration of an optical receiving device disclosed in Non-Patent Literature 1. As illustrated in FIG. 5, an optical receiving device 60 includes an optical receiver 61, an analog-to-digital converter (ADC) 62, a Stokes vector calculation circuit 63, a tap-coefficient-using Stokes vector calculation circuit 64, a tap coefficient updating circuit 65, a complex symbol calculation circuit 66, a carrier phase estimation circuit 67, and a symbol determination circuit 68. In this optical receiving device 60, the randomly fluctuating polarization state is tracked in a blind state at all times mainly by the tap-coefficient-using Stokes vector calculation circuit 64 and the tap coefficient updating circuit 65.
In the tap-coefficient-using Stokes vector calculation circuit 64 and the tap coefficient updating circuit 65, polarization fluctuation-is tracked by using a 3×2 multiple-input and multiple-output (MIMO) configuration so that, by inputting three Stokes parameters, two real numbers (or an imaginary number in which one of two real numbers constitutes a real part and the other constitutes an imaginary part) are output while updating tap coefficients as appropriate.
Specifically, in the tap-coefficient-using Stokes vector calculation circuit 64, an output Stokes vector is calculated using tap coefficients constituted by three complex numbers (or six real numbers). Next, the tap coefficients are updated by the tap coefficient updating circuit 65 in accordance with update formulas corresponding to respective algorithms. In updating the tap coefficients, a least mean square (LMS) algorithm or a constant modulus algorithm (CMA) is used, but when blind equalization is performed, it is required that CMA is used and that carrier phase estimation is subsequently performed.
In Non-Patent Literature 2, random polarization fluctuation is tracked by using a 3×1 multiple-input and single-output (MISO) configuration in which a complex signal is output by inputting three Stokes parameters, as in Non-Patent Literature 1. However, despite the “single-output”, this configuration is not essentially different from the 3×2 MIMO configuration in Non-Patent Literature 1 since dealing with a complex signal is substantially equivalent to dealing with two real numbers, in Non-Patent literature 2, LMS is adopted as the algorithm for updating tap coefficients, and there is no description about a method for blind equalization.